A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the manufacture of devices using lithographic processes, each mask pattern is typically projected onto the target portion in focus. In practice, this means that the target portion of the substrate is positioned in a plane of best focus of the aerial image projected by the projection system. As the critical dimension (CD), i.e. the dimension of a feature or features in which variations will cause undesirable variation in physical properties of the feature, such as the gate width of a transistor, in lithography shrinks, consistency of focus, both across a substrate and between substrates, becomes increasingly important.
The use of an alignment system to monitor focus has been proposed and involves printing focus-sensitive alignment markers at known positions relative to normal alignment markers at various different focus settings, i.e. positions of the substrate relative to the projection system. The position of these focus-sensitive markers with respect to the normal alignment markers is measured and an alignment offset (AO) can be determined which is representative of focus errors.
The quality of focus control in lithographic tools is today verified by using the Leveling Verification Test (LVT). A potential benefit of this method is that the read-out of the wafer can be done by the alignment system present on the lithographic tool itself. Hence no off-line read-out tool may be needed. The LVT test uses a special reticle with glued glass wedges on top, to locally create non-telecentric illumination on a double telecentric lens. This non-telecentric illumination is used to cause a lateral shift in x, y as function of defocus z of the aerial image of an XPA alignment mark situated beneath a glass wedge. By measuring the alignment shift of this defocus mark with respect to XPA reference mark (imaged without wedge on top), the defocus at the moment of exposing can be determined.
Up to this point, the current LVT test has been working quite well. However, for future lithographic projection tool designs, three potential drawbacks of the LVT method may become relevant. The tighter focus control for new systems put higher demands on the signal to noise ratio of the defocus measurement technique. The read-out noise of the alignment system, and the positioning accuracy of the reference marks are important contributors to the measurement noise of LVT, due to a fairly low focus-versus alignment shift sensitivity (typically d(X,Y)/dZ=0.4). This low focus versus shift sensitivity can not be further increased due to restriction of the height and angle of wedges). Secondly, the LVT test has a restricted spatial sampling density due to the fact that each defocus measuring mark requires a relatively large wedge on top. Finally, and more importantly, the current LVT test method requires light being transmitted through wedges. The current LVT test method can thus not be applied for future maskless or EUV systems.